The present invention relates to a color image forming apparatus in which different color toner images are repeatedly superimposed in order to form a color image, an especially to a color image forming apparatus which is used in the field of electrostatic recording and electrophotography.
Conventionally, in order to form a color image according to, for example, an electrophotographing method, processes of charging, image exposing, developing, and transferring ar e repeated for each component color, and color toner images are superimposed on a recording sheet. That is, by using light which is modulated by color information from a color original document, the above-described processes are repeated four times and color toner images are obtained so that a color image formed by yellow, magenta, cyan and black toners can be formed.
However, in the above-described color image forming method, it is necessary to transfer one color toner image onto a transfer body each time after development has been completed for each color toner. Therefore, problems arise such as the dimensions of the apparatus must be increased, and the recording process becomes complicated, so that a large amount of time is wasted. Further, since each color toner image is transferred onto a recording sheet after each process, transfer slippage occurs, and thus the quality of the color image is decreased. In order to solve the foregoing problems, a color image forming method is known, in which a plurality of toner images are superimposed onto the same photoreceptor so that a transferring process can be completed at one time.
However, in the above-described prior art method, for example, yellow, magenta, cyan and black toner images are superimposed, to form a color image. However, it is more difficult to make black toner adhere to the toner surface than the other toners. That is, as shown in FIG. 1 (see especially the edge effect), for example, a yellow toner, (which is the first color toner in this process), forming a toner image 100 can easily be made to adhere onto an edge portion. In FIG. 1, each when image exposing is used in order to make a toner image 101 it adheres to the toner surface (as shown by a two-dotted chain line) when magenta, cyan, and black toner images are subsequently superimposed onto the toner surface in the same way as the yellow toner image and after the yellow toner image is made to adhere the photoreceptor surface, it is difficult to make the toner image 101 adheres an edge portion after the yellow color toner adheres to the toner surface. Thus, it becomes more difficult to superimpose one toner image onto the surface of another toner image as the number of superimposing processes advances further. Furthermore, since the toner image 100 is exposed from the upper portion as shown in FIG. 3, an edge portion of the image such as, for example, a character becomes thicker because toner splashes occur when the toner image is exposed again.
As described above, when a later development is performed, the toner is difficult to make toner adhere onto an edge portion of the toner image obtained by the former development, and further, the toner is disturbed, so that a hue of the image is changed, or the image can be splattered with toner.